1. FIELD OF THE INVENTION
The field of the invention is catalytic cracking of heavy hydrocarbon feeds in general, and reduction of NO.sub.x emissions from the regenerator associated with the cracking unit.
2. DESCRIPTION OF RELATED ART
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425C-600C, usually 460C-560C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen-containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500C-900C, usually 600C-750C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most FCC units now use zeolite-containing catalyst having high activity and selectivity. These catalysts work best when the amount of coke on the catalyst after regeneration is relatively low. It is desirable to regenerate zeolite catalysts to as low a residual carbon level as is possible. It is also desirable to burn CO completely within the catalyst regenerator system to conserve heat and to minimize air pollution. Heat conservation is especially important when the concentration of coke on the spent catalyst is relatively low as a result of high catalyst selectivity. Among the ways suggested to decrease the amount of carbon on regenerated catalyst and to burn CO in the regenerator is to add a CO combustion promoter metal to the catalyst or to the regenerator. Metals have been added as an integral component of the cracking catalyst and as a component of a discrete particulate additive, in which the active metal is associated with a support other than the catalyst. U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121, incorporated herein by reference, introduced relatively large-sized particles containing CO combustion-promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, of small-sized catalyst particles, cycled between the cracking reactor and the catalyst regenerator, while the combustion-promoting particles remain in the regenerator. Oxidation-promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated on an inorganic oxide such as alumina, are disclosed.
U.S. Pat. No. 4,072,600 and U.S. Pat. No. 4,093,535 teach use of combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Many FCC units use CO combustion promoters. This reduces CO emissions, but usually increases nitrogen oxides (NO.sub.x) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NO.sub.x content of the regenerator flue gas.
SOx emissions are also a problem in many FCC regenerators. SOx emissions can be greatly reduced by including a SOx capture additive in the catalyst inventory, and operating the unit at relatively high temperature, in a relatively oxidizing atmosphere. In such conditions, the SOx additive can adsorb or react with SOx in the oxidizing atmosphere of the regenerator, and release the sulfur as H2S in the reducing atmosphere of the cracking reactor. Platinum is known to be useful both for creating an oxidizing atmosphere in the regenerator via complete CO combustion and for promoting the oxidative adsorption of SO2. Hirschberg and Bertolacini reported on the catalytic effect of 2 and 100 ppm platinum in promoting removal of SO2 on alumina. Alumina promoted with platinum is more efficient at SO2 removal than pure alumina without any platinum. Unfortunately, those conditions which make for effective SOx removal (high temperatures, excess 02, Pt for CO combustion or for SOx adsorption) all tend to increase NO.sub.x emissions.
Many refiners have recognized the problem of NO.sub.x emissions from FCC regenerators, but the solutions proposed so far have not been completely satisfactory. Special catalysts have been suggested which hinder the formation of NO.sub.x in the FCC regenerator, or perhaps reduce the effectiveness of the CO combustion promoter used. Process changes have been suggested which reduce NO.sub.x emissions from the regenerator.
Recent catalyst patents include U.S. Pat. No. 4,300,997 and its division U.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO combustion promoter. The bi-metallic CO combustion promoter is reported to do an adequate job of converting CO to CO2, while minimizing the formation of NO.sub.x.
Another catalyst development is disclosed in U.S. Pat. No. 4,199,435 which suggests steam treating conventional metallic CO combustion promoter to decrease NO.sub.x formation without impairing too much of the CO combustion activity of the promoter.
U.S. Pat. No. 4,235,704 suggests too much CO combustion promoter causes NO.sub.x formation, and calls for monitoring the NO.sub.x content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NO.sub.x in the flue gas. As an alternative to adding less CO combustion promoter the patentee suggests deactivating it in place, by adding something to deactivate the Pt, such as lead, antimony, arsenic, tin or bismuth.
Process modifications are suggested in U.S. Pat. No. 4,413,573 and U.S. Pat. No. 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NO.sub.x emissions.
U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NO.sub.x emissions.
U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel to the upper portion of a FCC regenerator to minimize NO.sub.x emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator.
The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
All the catalyst and process patents discussed above from U.S. Pat. No. 4,300,997 to U.S. Pat. No. 4,542,114, are incorporated herein by reference.
In addition to the above patents, there are myriad patents on treatment of flue gases containing NO.sub.x. The flue gas might originate from FCC units, or other units. U.S. Pat. No. 4,521,389 and U.S. Pat. No. 4,434,147 disclose adding NH3 to NO.sub.x containing flue gas to catalytically reduce the NO.sub.x to nitrogen.
None of the approaches described above provides the perfect solution. Process approaches, such as multi-stage or countercurrent regenerators, reduce NO.sub.x emissions but require extensive rebuilding of the FCC regenerator.
Various catalytic approaches, e.g., use of bi-metallic CO combustion promoters, steamed combustion promoters, etc., to degrade the efficiency of the Pt function help some but still may fail to meet the ever more stringent NO.sub.x emissions limits set by local governing bodies.
I discovered a way to use a zinc-based catalyst to reduce NO.sub.x emissions in the flue gas from the regenerator.